Commonly, it is known that the solid surface characteristics are improved by forming a thin film on or injecting ions into the surface of a solid material in plasma. A film formed using plasma containing metal ion or nonmetal ion strengthens the abrasion resistance/corrosion resistance of the solid surface, and it is useful as a protective film, an optical film, a transparent electroconductive film and such. In particular, a carbon film using carbon plasma has a high utility value, as diamond-like carbon film (denoted as “DLC”) formed from diamond and graphite structures.
As a method for generating plasma containing metal or nonmetal ion, there is a vacuum arc plasma method. Vacuum arc plasma is formed in an arc discharge occurring between a cathode and an anode, where the cathode material evaporates from an existing cathode ray spot on the cathode surface, and it is plasma formed by this vaporized cathode material. Also, when a reactive gas is introduced as the ambient gas, the reactive gas is ionized simultaneously, too. An inert gas (denoted as “noble gas”) may be introduced along with said reactive gas, and also, said inert gas can be introduced in place of said reactive gas. By means of such plasma, a surface treatment can be done by thin film formation or ion injection to a solid surface.
Normally, by vacuum arc discharge, from cathode spots, vacuum arc plasma constituent particles are is ejected, such as cathode material ions, electrons, and cathode material neutral atom groups (atoms and molecules). At the same time, cathode material particles, referred to as droplets, with size ranging from less than submicron and up to several hundred microns (0.01-1000 μm), are also ejected. When these droplets adhere to the surface of the object to be treated, the uniformity of the film formed on the surface of the object to be treated is lost, defects of the thin film are caused, and an effect is caused on the surface treatment result of the film formation.
In a plasma processing apparatus using a vacuum arc plasma method, for example, as present applicant already disclosed in Japanese Patent Laid-Open No. 2008-91184 bulletin (Patent Document 1), plasma is generated by inducing an electric spark between the cathode and the trigger electrode, and generating a vacuum arc between the cathode and the anode.
[Patent Document 1] Japanese Patent Laid-Open No. 2008-91184 bulletin